A standard platen press has upper and lower platens each provided with at least one rigidifying beam. Means such as a group of powerful hydraulic cyliners spaced about over one of the platens serves to urge the two platens together in a pressing direction to compress a workpiece--plywood, fiber mat, chipboard, or the like--between the platens. During pressing it is inevitable that the platens and their rigidifying beams bend somewhat. It has been found impractical to use beams so rigid that they do not bend at all under the normal relatively high pressing forces; such platens would be unusably large and heavy.
The faces of the platens must be parallel during the pressing operation, however, in order to produce a product whose faces are parallel, as is invariably required. The simplest way of achieving this planarity is to use press platens that are bowed slightly to start with, but that flatten out to a planar shape when deformed during pressing, or to use an appropriately shaped insert between the platen beams and the platen.
Recent times have, however, required that the workpieces be to very high tolerances. The prior-art system of nonplanar platens cannot meet such requirements, so recourse has been had to various systems for measuring the platen deformation so that the pressure in the appropriate cylinders can be increased to hold the platens in planar shape.
German patent document 1,703,297 filed Apr. 29, 1968 under serial P1,703,297.2 describes a complex system wherein actuation cylinders are distributed over the surface of the platen. This system has a plurality of reference bars extending parallel to the platens and immediately behind them. These reference bars are supported only at their ends and are normally perfectly straight. Each engages a plurality of sensors carried on the respective platen and connected to respective valves that control respective compensating cylinders. When a platen bends so that at one region it, for instance, approaches the bar, the respective sensor operates the respective valve to change pressure in the respective compensating cylinder to compensate out the deformation.
Such an arrangement is quite complex and has not shown itself to operate with the high degree of accuracy, plus or minus 1 micron, it is supposed to have. The bars frequently are deformed thermally in the normally heated press, and often droop with time so that the readings they give become meaningless. The mountings at their ends are not without friction, so that as the platens deform some of this deformation is transmitted to these bars, worsening the accuracy of the system. The considerable expense of this system, coupled with it relatively poor accuracy, has mitigated against its widespread use.